It is common to measure the electrical potentials present on the interior surface of the heart as a part of an electrophysiologic study of a patient's heart. Typically such measurements are used to form a two-dimensional map of the electrical activity originating from the heart. An electrophysiologist will use the map to locate centers of ectopic electrical activity occurring within cardiac tissue.
One traditional mapping technique involves a sequence of electrical measurements taken from mobile electrodes inserted into the heart chamber. The electrodes are repetitively moved into contact with the endocardial surface.
An alternative mapping technique takes essentially simultaneous measurements from a floating electrode array to generate a two-dimensional map of electrical potentials. This non-contact technique is taught by a number of references including Taccardi U.S. Pat. No. 4,649,924.
The two-dimensional maps of the electrical potentials at the endocardial surface generated by these traditional processes suffer many defects. One defect is related to "spatial averaging". Traditional systems have been limited in resolution by the number of electrodes used. The number of electrodes dictated the number of points for which the electrical activity of the endocardial surface could be mapped. Therefore, progress in endocardial mapping has involved either the introduction of progressively more electrodes on the mapping catheter or improved flexibility for moving a small mapping probe with spot electrodes from place to place on the endocardial surface. Both "ring" shaped and "spot" shaped electrodes are constrained by spatial averaging within the blood volume in the heart chamber. The ring shaped electrode spatially averages the measurement of electrical activity around the circumference of the electrode site. The spot electrode also spatially averages the measurement of electrical activity within the conical view of the electrode site. Direct contact with electrically active tissue is required by most systems in the prior art in order to minimize this limitation of circumferential spatial averaging of the ring electrode or conical spatial averaging of the spot electrode. When a ring shaped electrode is in contact with the electrically active tissue, the local electrical potential at the point of contact is directly coupled to the electrode and the contact potential dominates over the effect of circumferential spatial averaging of the electric field. Thus the problem of spatial averaging is the inability to accurately resolve the location of ectopic tissue masses. In the prior art, iso-potentials are interpolated and plotted on a rectilinear map which can only crudely represent the unfolded interior surface of the heart. Such two-dimensional maps are generated by interpolation processes which "fill in" contours based upon a very limited set of measurements. They are not three-dimensional and thus cannot be used to distinguish a large ectopic center located a long distance from an electrode site, from a smaller ectopic centers located closer to the electrode site. The traditional twodimensional mapping process also cannot locate infarcted tissue which is electrically inactive. The inability to accurately characterize the size and location of ectopic tissue frustrates the delivery of certain therapies such as "ablation".